Dijkstra's algorithm in PHP

Question

This is Dijkstra's algorithm in PHP. Regarding the data structure, I use a 2-dimensional array $_distArr[m][n]=length to store it. Are there any improvements for this?

$a and $b are the start and the end node, respectively.

<?php
/**
 * Dijkstra's algorithm in PHP by zairwolf
 * 
 * Demo: http://www.you4be.com/dijkstra_algorithm.php
 *
 * Source: https://github.com/zairwolf/Algorithms/blob/master/dijkstra_algorithm.php
 *
 * Author: Hai Zheng @ https://www.linkedin.com/in/zairwolf/
 *
 */
//set the distance array
$_distArr = array();
$_distArr[1][2] = 7;
$_distArr[1][3] = 9;
$_distArr[1][6] = 14;
$_distArr[2][1] = 7;
$_distArr[2][3] = 10;
$_distArr[2][4] = 15;
$_distArr[3][1] = 9;
$_distArr[3][2] = 10;
$_distArr[3][4] = 11;
$_distArr[3][6] = 2;
$_distArr[4][2] = 15;
$_distArr[4][3] = 11;
$_distArr[4][5] = 6;
$_distArr[5][4] = 6;
$_distArr[5][6] = 9;
$_distArr[6][1] = 14;
$_distArr[6][3] = 2;
$_distArr[6][5] = 9;
//the start and the end
$a = 1;
$b = 5;
//initialize the array for storing
$S = array();//the nearest path with its parent and weight
$Q = array();//the left nodes without the nearest path
foreach(array_keys($_distArr) as $val) $Q[$val] = 99999;
$Q[$a] = 0;
//start calculating
while(!empty($Q)){
 $min = array_search(min($Q), $Q);//the most min weight
 if($min == $b) break;
 foreach($_distArr[$min] as $key=>$val) if(!empty($Q[$key]) && $Q[$min] + $val < $Q[$key]) {
 $Q[$key] = $Q[$min] + $val;
 $S[$key] = array($min, $Q[$key]);
 }
 unset($Q[$min]);
}
//list the path
$path = array();
$pos = $b;
while($pos != $a){
 $path[] = $pos;
 $pos = $S[$pos][0];
}
$path[] = $a;
$path = array_reverse($path);
//print result
echo "<img src='http://www.you4be.com/dijkstra_algorithm.png'>";
echo "<br />From $a to $b";
echo "<br />The length is ".$S[$b][1];
echo "<br />Path is ".implode('->', $path);

• Potherca: Could you define what kind of "improvement" you are hoping to find? At first glance there are quality, readability and speed improvements possible for this piece of code..
• coderodde: Looks really pretty.

Answer 1

This implementation is great and works well, but you should add an "escape door" when no path is found.

For instance, remove these two lines:

$_distArr[4][5] = 6;
...
$_distArr[6][5] = 9;

If you comment both lines, you enter an infinite loop.

To avoid this problem, I think you can add these lines right before the "calculating while" loop:

if (!array_key_exists($b, $S)) {
 echo "Found no way.";
 return;
}

Then you're set; no infinite loop!

• SirPython: Nice review! Just for the future, answers should not be left in comments; answers are meant for answers. (I am mentioning this because you wrote "I cannot comment so I'm answering").

Answer 2

Implementation works well. Please note, your code assumes all edges are bi-directional with the statement below:

foreach(array_keys($_distArr) as $val) $Q[$val] = 99999;

If you add $_distArr[6][7] = 1; and change $b = 5; to $b = 7, then you will get an error because 7 is not inserted in $Q.